真核生物转录起始影响因子的研究进展

转录水平的调控是基因表达调控的主要方式。本文从染色质与转录激活、RNA聚合酶Ⅱ复合体以及转录起始因子等方面综述了目前关于转录起始影响因子的一些研究进展。     

真核生物转录起始影响因子的研究进展

转录水平的调控是基因表达调控的主要方式．本文从染色质与转录激活、ＲＮＡ 聚合酶Ⅱ复合体以及转录起始因子等方面综述了目前关于转录起始影响因子的一些研究进展．     

Cloning of cDNA for the glutamate-binding subunit of an NMDA receptor complex.

The amino acids L-glutamic and L-aspartic acids form the most widespread excitatory transmitter network in mammalian brain. The excitation produced by L-glutamic acid is important in the early development of the nervous system, synaptic plasticity and memory formation, seizures and neuronal degeneration. The receptors activated by L-glutamic acid are a target for therapeutic intervention in neurodegenerative diseases, brain ischaemia and epilepsy. There are two types of receptors for the excitatory amino acids, those that lead to the opening of cation-selective channels and those that activate phospholipase C (ref. 11). The receptors activating ion channels are NMDA (N-methyl-D-aspartate) and kainate/AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate)-sensitive receptors. The complementary DNAs for the kainate/AMPA receptor and for the metabotropic receptor have been cloned. We report here on the isolation and characterization of a protein complex of four major proteins that represents an intact complex of the NMDA receptor ion channel and on the cloning of the cDNA for one of the subunits of this receptor complex, the glutamate-binding protein.     

Malignant transformation by a eukaryotic initiation factor subunit that binds to mRNA 5' cap

Eukaryotic cellular mRNAs have a 5' cap structure (m7 GpppX) that facilitates binding to ribosomes and is required for efficient translation. A specific initiation factor, eIF-4F, mediates the function of the cap and consists of three subunits, one of which, eIF-4E, binds the cap. This subunit is present in limiting amounts in the cell, and is thought to be regulated by phosphorylation: decreased phosphorylation of eIF-4E following various treatments correlates with a decrease in cellular translation rate. These observations suggest that eIF-4E lies on the mitogenic signal transduction pathway, and we reasoned that overexpression of eIF-4E might profoundly affect cellular growth properties. We report here that overexpression of eIF-4E in NIH 3T3 and Rat 2 fibroblasts causes their tumorigenic transformation as determined by three criteria: formation of transformed foci on a monolayer of cells; anchorage-independent growth; and tumour formation in nude mice.     

Characterization of cDNA and genomic clones encoding the precursor to rat hypothalamic growth hormone-releasing factor

Growth hormone-releasing factor (GHRF) is a hypothalamic peptide which positively regulates the synthesis and secretion of growth hormone in the anterior pituitary. The amino-acid sequence of a 43-residue GHRF peptide isolated from rat hypothalamus was recently determined. Immunocytochemical techniques have been used to localize GHRF-containing cell bodies and nerve fibres largely to the medial-basal region of the rat hypothalamus. The rat has also been used extensively as an animal model to study the effects of GHRF on growth hormone synthesis and secretion and on somatic growth. To pursue questions concerning the biosynthesis of GHRF, the expression of the ghrf gene, and its regulation in the hypothalamus by neural and hormonal influences, we have now isolated and characterized both complementary DNA and genomic clones encoding rat hypothalamic GHRF. The rat ghrf gene spans nearly 10 kilobases (kb) of rat genomic DNA, contains 5 exons and encodes a 104-amino-acid precursor to the rat GHRF peptide. Comparison with previously characterized human ghrf cDNA and genomic clones has allowed patterns of conservation of amino-acid and nucleotide sequences between the human and rat GHRFs to be determined.     

Cloning of a cDNA for a glutamate receptor subunit activated by kainate but not AMPA.

Fast excitatory transmission in the vertebrate central nervous system is mediated mainly by L-glutamate. On the basis of pharmacological, physiological and agonist binding properties, the ionotropic glutamate receptors are classified into NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate) and kainate subtypes. Sequence homology between complementary DNA clones encoding non-NMDA glutamate receptor subunits reveals at least two subunit classes: the GluR1 to GluR4 class and the GluR5 class. Here we report the cloning and expression of a functional rat glutamate receptor subunit cDNA, GluR6, which has a very different pharmacology from that of the GluR1-GluR4 class. Receptors generated from the GluR1-GluR4 class have a higher apparent affinity for AMPA than for kainate. When expressed in Xenopus oocytes the homomeric GluR6 receptor is activated by kainate, quisqualate and L-glutamate but not by AMPA, and the apparent affinity for kainate is higher than for receptors from the GluR1-GluR4 class. Desensitization of the receptor was observed with continuous application of agonist. The homomeric GluR6 glutamate receptor exhibits an outwardly rectifying current-voltage relationship. In situ hybridizations reveal a pattern of GluR6 gene expression reminiscent of the binding pattern obtained with [3H]kainate.     

Characterization and expression of a cDNA, AmphiSDHD,encoding the amphioxus cytochrome b small subunit in mitochondrial succinate-ubiquinone oxidoreductase

In this study, an amphioxus cDNA, AmphiSDHD, encoding the cytochrome b small subunit in mitochondrial succinate-ubiquinone oxidoreductase, was isolated from the gut cDNA library of amphioxus Branchiostoma belcheri tsingtauense. It is 1429 bp in length, with an open reading frame of 465 bp coding for a protein of 154 amino acids. The deduced protein contains a mitochondrial targeting presequence of 65 amino acids rich in basic residues like arginine and hydroxy residues such as serine and threonine. Alignment of the amino acid sequences of AmphiSDHD and other eukaryotic SDHD proteins showed that AmphiSDHD has three transmembrane segments, and includes two histidine residues in the second transmembrane segment that are the putative binding sites for the heme b molecule. The phylogenetic tree constructed suggests that AmphiSDHD appears more closely related to vertebrate SDHD proteins than invertebrate ones. Northern blotting demonstrated that AmphiSDHD is ubiquitously expressed in amphioxus, being in line with the fact that SDHD is a house-keeping protein.     

Cloning and structure of a yeast gene encoding a general transcription initiation factor TFIID that binds to the TATA box

The TATA sequence-binding factor TFIID plays a central role both in promoter activation by RNA polymerase II and other common initiation factors, and in promoter regulation by gene-specific factors. The sequence of yeast TFIID, which seems to be encoded by a single gene, contains interesting structural motifs that are possibly involved in these functions, and is similar to sequences of bacterial sigma factors.